Many devices and systems employ moving components that are supported in noncontacting relationship with respect to each other. Such an arrangement is particularly advantageous where the frictional force between the components is to be minimized. Without a mechanical connection between the components, however, some form of nonmechanical sensor must be provided if information concerning the gap between, and relative position of, the components is to be produced.
Turning to one example of particular interest, some transportation systems employ noncontacting elements to enhance system speed, safety, and efficiency. Specifically, such transportation systems involve the suspension of a vehicle above or below a roadway along which the vehicle is propelled. By eliminating frictional force between the vehicle and roadway, energy losses are reduced, as are system maintenance requirements. In addition, because the vehicle is not mechanically coupled to the roadway, surface conditions attributable to factors such as weather will not affect vehicle performance, enhancing system safety and resulting in more consistent vehicle ride characteristics.
The controlled suspension of the vehicle with respect to the roadway is often achieved through the use of magnetic force. For example, the magnetic repulsion or attraction between magnets located on both the roadway and the vehicle can be used for suspension. Alternatively, the attractive force between magnets and a magnetically permeable material, such as iron, placed in an opposing relationship in the vehicle and roadway, can be used to eliminate contact therebetween. For convenience, systems employing magnetic suspension also typically employ any of a variety of magnetic motor arrangements to propel the vehicle along the roadway.
As will be appreciated, the control of such transportation systems requires information regarding both the "gap" between the vehicle and roadway (e.g., the spacing between the vehicle and roadway measured in a direction normal to the path of the vehicle's travel) and the "relative position" of the vehicle with respect to the roadway (e.g., the position of the vehicle relative to the roadway measured in a direction parallel to the path of vehicle travel). For example, the length of the gap between the vehicle and roadway may affect the safety, operating characteristics, and efficiency of the transportation system. The gap length, however, is a function of a number of factors, including both the energy applied to the magnetic suspension system and the load applied to, and operation of, the vehicle. Thus, feedback concerning the influence of system operating conditions on gap length is helpful to maintain the desired separation between the vehicle and roadway.
Like gap length, information concerning the relative position of the vehicle with respect to the roadway is useful to maintain control of the transportation system. For example, this information identifies vehicle location, and, when combined with timing information, can be used to determine vehicle velocity and acceleration. Analysis of the position, velocity, and acceleration information can be used to provide feedback to the vehicle propulsion system to control each of these parameters of transportation system operation, as desired.
One prior art device used for performing measurements between noncontacting components is commonly referred to as a "Hall-effect" device or sensor. Such a device is made of a semiconducting material having characteristics that result in the production of an electric potential along a first axis when an electric current is applied along a second axis, normal to the first, and a magnetic field is applied along a third axis, normal to the first and second axis. While the resultant "Hall" potential is a function of a number of characteristics of the device, it is also proportional to the strength of both the applied electric current and magnetic field. As a result, if the characteristics of the device and the magnitude of the applied current are known, the Hall potential can be used to indicate the strength of the magnetic field applied.
Such Hall-effect devices have been used to measure various characteristics between noncontacting elements. For example, as shown in U.S. Pat. No. 3,419,798 (Walton), a pair of transducers employing Hall-effect devices are used to measure the varying gap between two objects that are relatively displaceable in a direction normal to the gap. Because the gap is a function of only the relative displacement of the two objects, measurement of the gap leads directly to an indication of displacement. Similarly, U.S. Pat. No. 2,987,669 (Kallmann) discloses several embodiments of a system employing the Hall-effect to monitor the motion of two elements in a direction normal to their fixed separating gap.
Such prior art Hall-effect arrangements conveniently allow either gap or relative displacement to be measured in systems where the unmeasured quantity is held constant. Alternatively, both gap and displacement can be determined if a fixed relationship is previously known to exist between the two variables. Prior art arrangements have not, however, been able to produce an output indicating both gap and relative displacement when both quantities vary in an unrelated manner.
As noted previously, information regarding both these variables is required for the proper operation of transportation systems employing, for example, magnetic suspension and propulsion to maintain a noncontacting relationship between the vehicle and roadway. In such systems, gap and displacement each vary in an unrelated manner. Further, because transportation systems of this type typically include roadways comprised in part of spaced-apart pole pieces, the "apparent" gap may abruptly alternate between relatively high and low levels as the sensing device alternatively traverses regions of the roadway having low and high magnetic permeabilities. In light of the preceding remarks, it would be desirable to provide a noncontacting sensing system that produces an output indicative of both gap and relative displacement, regardless of the manner in which these variables change.
A second problem not previously addressed by the art and of particular applicability to the use of Hall-effect devices, involves the variability of the Hall potential as a function of factors other than gap or relative position. For example, fluctuations in the operation of the source of the magnetic field, as well as variations in the temperature of the Hall device, may alter the device's output. Without some form of compensation, an erroneous determination of gap or relative position would occur. It would, therefore, be desirable to produce a sensing system employing a Hall-effect sensor whose potential is unaffected by such variables.